Plant architecture influences gall abundance in a tropical montane plant species

Sessile and host-specific herbivores, such as gall-inducing insects, are usually patchily distributed within the populations of their host plants, and it has been suggested that both inter- and intra -plant variation affect gall abundance, distribution and survivorship. Variation in plant traits, such as size and architecture, has been previously demonstrated as a determinant of gall distribution. We examined the influence of architectural complexity of the tropical plant Galianthe brasiliensis (Rubiaceae) on the abundance of the stem-galler Lopesia sp. (Diptera, Cecidomyiidae) along an elevation gradient where both insect and plant were distributed. A total of 120 plants were sampled along the gradient and more than 580 galls were counted, while plant architectural complexity was determined by a combination of height and branch ramification. Increased elevation did not influence plant complexity nor gall abundance (both P>0.05), but plant architectural complexity explained more than 60% of the variation in gall abundance along the gradient (R-2=0.62, P< 0.001). We suggest that the greater availability of meristematic tissues in more architecturally complex plants explains the results found, as this is a key resource for gall occurrence and establishment.